Genetics
- Monohybrid cross (Aa × Aa)
- genotypes 1 : 2 : 1, phenotypes 3 : 1
- Dihybrid cross
- phenotypes 9 : 3 : 3 : 1
- Hardy–Weinberg
- p + q = 1, p² + 2pq + q² = 1
- Independent genes
- multiply the probabilities
Biology: Genetics and Cell Biology: cram pack
Formula sheets, the key method steps for every topic and 4 practice problems, with full solutions at the back.
1Formulas and methods
2Practice problems
1.GeneticsMedium
Tall (A) is dominant over short (a), and round seeds (B) over wrinkled (b). The genes are on different chromosomes. For the cross AaBb × Aabb, what is the probability P that an offspring is tall, wrinkled seeds?
AaBb × Aabb
P =
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives round seeds (dominant) or wrinkled seeds; gene B gives purple flowers (dominant) or white flowers. Click the one cell for an offspring that is homozygous for round seeds and shows white flowers.
AaBb × AaBb
- 1ab × ab
- 2ab × Ab
- 3ab × AB
- 4ab × aB
- 5AB × ab
- 6AB × Ab
- 7AB × AB
- 8AB × aB
- 9Ab × ab
- 10Ab × Ab
- 11Ab × AB
- 12Ab × aB
- 13aB × ab
- 14aB × Ab
- 15aB × AB
- 16aB × aB
3.Cell biologyMedium
Which motor protein generally carries cargo along microtubules towards the minus end, near the cell centre?
- AKinesin
- BDynein
- CActin
- DMyosin
4.Cell biologyHard
Non-disjunction during meiosis can result in:
- AGametes with an abnormal number of chromosomes, such as in Down syndrome
- BIdentical twins
- CCrossing over
- DMitochondrial DNA mutations only, which are passed on through the egg cytoplasm
3Solutions
1.Genetics
Answer: P = 0.375
Mendelian genetics: each parent passes on one allele of each gene, chosen at random. A Punnett square lists every combination of gametes, each equally likely. A dominant allele (capital letter) shows whenever it is present; a recessive one only in the homozygous form. Genes on different chromosomes are inherited independently, so their probabilities multiply.
Each gene on its own
P(tall) = 0.75
P(wrinkled seeds) = 0.5
Multiply
P = 0.75·0.5 = 0.375
Answer
P = 0.375
2.Genetics
Answer: Ab × Ab
Punnett square: one parent's gametes run along the top, the other's down the side. Each cell joins the gamete above it with the gamete beside it, and every cell is equally likely. A recessive trait only shows when both alleles are recessive; for an X-linked trait, sons get their only X from the mother.
Genotype asked for
AAbb
Gametes
A homozygous genotype needs the same gamete from both parents, so only one cell in the 16 fits.
Ab × Ab = AAbb
Answer
Cell Ab × Ab → AAbb
3.Cell biology
Answer: B · Dynein
Why this is the answer
Kinesins mostly move towards the plus end (out to the periphery) and dyneins towards the minus end (towards the centrosome). Myosin moves along actin.
Answer
B · Dynein
4.Cell biology
Answer: A · Gametes with an abnormal number of chromosomes, such as in Down syndrome
Why this is the answer
Failure of homologues (meiosis I) or sister chromatids (meiosis II) to separate creates aneuploid gametes, e.g. trisomy 21.
Answer
A · Gametes with an abnormal number of chromosomes, such as in Down syndrome
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